Use rational representation if possible (#182)

Author: YingboMa

src/time_integration_methods.jl

@@ -119,8 +119,10 @@ Reference: Section 315 of
 function residual_order_condition(t::RootedTree, rk::RungeKuttaMethod)
     ew = elementary_weight(t, rk)
     T = typeof(ew)
+    invγ = 1//γ(t)
+    invσ = 1//σ(t)
 
-    (ew - one(T) / γ(t)) / σ(t)
+    (ew - invγ) * invσ
 end
 
 # TODO: Deprecations introduced in v2